Determination of engine air-fuel ratios may be made by one or more oxygen sensors located in the exhaust stream of the engine, and fuel injection amounts to the cylinders can be adjusted in response to the determined air-fuel ratio. However, the exhaust may contain multiple constituents, such as CO, H2, and unburnt hydrocarbons, and some of these constituents can bias the reading of the oxygen sensors. For example, aromatic hydrocarbons present in the exhaust, such as toluene, are known to bias oxygen sensors rich, interfering with accurate determination of the air-fuel ratio. Traditional solutions to account for aromatic hydrocarbons in the exhaust have included a lambda offset, whereby the calculated air-fuel ratio may be adjusted based on estimated aromatic hydrocarbon amounts as determined by engine speed, load, and cam position.
The inventors herein have identified a potential issue with the above approach. The amount of cyclic hydrocarbons produced by an engine may vary based on engine temperature. Further, the above approach does not factor in power-train to power-train variabilities and fuel differences among vehicles.
Thus, in one example, the above issue may be at least partially addressed by an engine exhaust system method. The method comprises adjusting a sensor calibration correction value of an exhaust sensor upstream of a catalyst based on an exhaust sensor downstream of the catalyst in response to steady-state conditions with engine temperature below a threshold while catalyst activity is above a threshold.
For example, the engine may be operating below normal operating temperature. As such, additional aromatic hydrocarbons may be present in the exhaust upstream of the catalyst, which can result in a biased sensor reading. If the catalyst is active, the hydrocarbons present in the exhaust stream will be oxidized in the catalyst. Thus, the sensor reading downstream of the catalyst is less likely to be biased by the presence of aromatic hydrocarbons. By adjusting a sensor calibration correction value of the upstream sensor based on the downstream sensor reading under conditions where both sensors should be reading the same oxygen level (or the same air-fuel ratio), the bias of the upstream sensor reading by the aromatic hydrocarbons may be identified, and used to provide accurate determination of the air-fuel ratio by the upstream sensor, even under cold engine operating conditions, thus improving fuel economy and decreasing emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.